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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine: ((1 << A) >>u B) --> (1 << (A-B)) incorrect if A-B > bitwidth"
href="http://llvm.org/bugs/show_bug.cgi?id=21274">21274</a>
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<th>Summary</th>
<td>InstCombine: ((1 << A) >>u B) --> (1 << (A-B)) incorrect if A-B > bitwidth
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>nunoplopes@sapo.pt
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</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>simplifyValueKnownNonZero() in InstCombineMulDivRem.cpp simplifies the RHS of
div/rem instructions.
However, the first transformation it does on RHS doesn't seem correct:
((1 << A) >>u B) --> (1 << (A-B))
Pre: isPowerOf2(%Power)
%shl = shl %Power, %A
%Y = lshr %shl, %B
%r = udiv %X, %Y
=>
%sub = sub %A, %B
%Y = shl %Power, %sub
%r = udiv %X, %Y
Alive says the following:
ERROR: Domain of definedness of Target is smaller than Source's for i4 %Y
Example:
%Power i4 = 0x4 (4)
%A i4 = 0x0 (0)
%B i4 = 0x1 (1)
%X i4 = 0x0 (0)
%shl i4 = 0x4 (4)
%sub i4 = 0xF (15, -1)
Source value: 0x2 (2)
Target value: undef
Can't see a fix other than removing this transformation altogether.</pre>
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